884化工原理的電子版本課件雙語(yǔ)chapter

1、1Chapter 6Agitation and mixing2Introductionapplication(1) dispersion of solvable solid(2) mixing and dispersion of immiscible liquids (3) mixing between gas and liquid(4) suspension of solid particles in liquid(5) acceleration of chemical reaction and physical transportagitation methodsmechanical ag

2、itatorsgas agitationjet mixing static mixertubular mixing3A set of mixing equipment consists of a mixing tank a driving motor with speed reducer an agitator some attached parts.Mixing equipment Agitator is the main part, like an impeller in a pump to give mechanical energy to liquid.4standard type：S

3、/d=1，Z=3 blade end speed: 515m/s，maximum 25m/s Types of agitatorspropeller Standard type：S/d=1B/d=0.1Z=1-2 (2 for twin ribbon type)low speed, the outer edge is very close to the tank wall. helical ribbon 5standard typed/B=4-10，Z=2blade end speed 1.53 m/s Types of agitatorsblades standard type： B/d=1

4、/12d/d=0.05-0.08, d=25-50 mmd- distance between the tank wall and the outer edge of the anchorblade end speed 0.5-1.5 m/s anchor and frame6straight vanes on diskcurve vanes on disk open straight vanesopen curve vanesTypes of agitatorsturbine 7Types of agitatorsaxial-flowThe main flow in tank is a ci

5、rculation on axial direction (& tangential) with little turbulent.suitable for mixing of low viscose liquids, particle suspension and heat transfer enhance.Propeller small diameter, high speed, large flow rate and low headHelical ribbon large diameter and mixing range, low speed, low head. Special d

6、esign for high viscosity liquid.It can be divided by flow pattern8Types of agitatorsRadial-flowComplicated radial and tangential flow.For low & middle viscosity liquids in dispersion of immiscible liquids, chemical reaction and heat transfer.turbine：high speed，wide blade，low flow rate and high head.

7、 straight vane：long vane, low speed and low head, for high viscosity liquid.anchor and frame ：very large diameter and mixing range, very low speed and head. Suitable for high viscosity liquids and capable of preventing the deposit on tank wall. 9Baffle and draft tubetangential vortex- by centrifugal

8、 force. The liquid level on tank center will fall to form a forced vortex. The high the speed , the deep the vortex. result effective volume reduced and mixing effect worsen. Sometimes gas is absorbed from lower liquid level to disturb operation.Solution 1 install baffles on tank wall.maximum 8 baff

9、les, called fully baffled10Baffles and draft tubedraft tube improving mixing through controlling the flow velocity and direction, reducing the short cut. Especially for particle suspension .Solution 2 off-central installed agitator will improve the operation with increased power consumption. 11Flow

10、pattern in mixing tank Stirring ReFlow pattern is related with the geometries of tank, stirrer and baffle, liquid properties and stirrer speed.For agitation operation, the useful flows are axial and radial, not the tangential. For a fully baffled standard tank with an 8 straight vane turbine, the fl

11、ow pattern 1 Re10 near the turbine: laminar flow, other zones: almost static Re10 laminar axis flow, flow starts from vane ends 100Re103 turbulent in whole tankdn = urotating speed, rpm12Mixing mechanism (1) molecular diffusion：occuring in molecular scale (2) turbulent diffusion：caused by vortex dis

12、sipation, existed in vortex size .(3) convective diffusion：caused by convection, occuring in large scale spaces.Convective flow breaks the liquid into large drops (macro mixing)；the drops are then broken into small drops by vortex deformation (inter-drop mixing)；those vortex breakage and deformation

13、 will increase or renew the contacting area between the drops with different concentration and promote the molecular diffusion. A fully homogeneous mixing depends on molecular diffusion.In comparison, the turbulent diffusion is about105107 times of molecular diffusion and dominates the turbulent agi

14、tation. 13Mixing mechanism of heterogeneous system Large vortex is broken into small ones by shearing effect. The viscose resistance converts part of the mixing energy into heat. Strong mixing effect occurs at the zone near the agitator. Total circulation flow rate is the most important for this typ

15、e of mixing. In the laminar zone, mixing depends on the total flow. But the agitator efficiency is low at turbulent zone. Large diameter and low speed agitators should be used.low viscosity liquidhigh viscosity liquid14Mixing mechanism of heterogeneous systemImmiscible liquid-liquid systemOne phase

16、is continuous and another is dispersed. For zone near the agitator, the shearing effect is strong under high turbulent and small liquid drops will be achieved. In the zone far away from the agitator, the drops will agglomerate into large ones. The breakage and agglomeration processes increase and re

17、new the interface of the liquids, so strengthen the inter-phase mass transfer. If a surface activation agent is added in this system, the agglomeration will be weaken and the size of liquid drops tends to be uniform.15Mixing mechanism of heterogeneous systemgas-liquid system The mechanism is similar

18、 to the liquid-liquid system. Gas is dispersed as bubbles in the liquid . The gas-liquid interface tension is stronger than that of liquid-liquid and the dispersion of gas is more difficult. As a result, the sizes of bubbles are larger than liquid drops. The large density difference between the gas

19、and liquid makes the gas bubbles rise to the top of the liquid. High shearing agitators are often used to generate relative small gas bubbles.16Mixing mechanism of heterogeneous system solid-liquid systemThe purpose of the agitation are to suspend the particles homogeneously in the liquid to reduce

20、the thickness of liquid film on particle surface in order to accelerate the reaction or transport processes.critical speed for suspension minimum rotating speed needed to suspend all particles. It is a function of agitator diameter and types.17Mixing effect Mixing effect can be expressed in differen

21、t ways. For chemical reaction, can be converted ratio of reactant. For heat and mass transfer, can be transport coefficients.I mixing index or percent. If we take n samples，then the average mixing index will beIf CACA0Let there are two liquids with volumes of VA and VB in the tank，the average concen

22、tration of A isI =1,?18Power of agitation pumping flow rate Q：flow rate pumped from the agitator (m3/s或m3/h)。circulating flow rate Q: all circulating flow rate in the tankby the entrainment from the agitator，Q Q.For turbulent flow (Re103)Pumping flow rate, head and power of agitatorFor turbulent flo

23、w, NQ is a constant, not a function of RePumping flow rate number NQ=Q/nd3circulating flow rate number NQ=Q/nd3H is proportional to the square of u,and u nd19Flow rate, head and power of agitator An agitator can be considered as a pump and the energy from its impeller will generate flow rate and hea

24、d to the liquid. The power can be A same N can give large flow rate and low head, or high head and low flow rate. Different process may need different Q and H . e. g., mixing of low viscosity liquids needs a large flow rate, but mixing of gas and liquid needs high turbulent flow or high head.With sa

25、me power, a large diameter and low speed agitator generates large flow rate. On the other hand, a small diameter and high speed agitator tends to generate a highly turbulent flow.20Correlations and power curves For a complicated mixing process, dimensional analysis is often used to correlate the exp

26、erimental data and find the empirical Eqs. With a standard mixing unit, following results can be found from the dimensional analysisP0 power numberRe stirring Reynolds number for flow patternFr Froude number for circulating flow with free surface“standard” mixing unit21Correlations and power curves

27、power functionConsidering the shapes of S1, S2,Sn , then function can be式中 k 為與流態(tài)區(qū)間有關(guān)，與幾何構(gòu)型有關(guān)的常數(shù)。 If there is no tangential vortex, the effect of Fr can be ignored, y = 0，and22Correlations and power curves Plot or P0 vs. Re on double logarithm coordinate，we obtain power curves. There is a certain cu

28、rve, no matter how large the size is, to correspond any one of agitator geometry.23Correlations and power curveslaminar Re 104Transition 10Re300，because of vortex，F(xiàn)r con not be ignored 、 are constants which related with the geometric factors, can be found in handbooks.agitator Const.24Design of agitation (1) Decide the type and geometry of the tank and the agitator.(2) Find the performance of the installation first, including the size, rotating speed and power, then scaling up to co